Yep. Saw on twitter that Isco is our top priority for the January window, and that on the other hand would be close to perfect IMO (for an attacking player). He would go in to the side that played Leicester where Mata played (with Mkhitaryan ideally replacing Lingard and Martial rotating with Rashford). Mata, as good as he plays, we all know won't be Mourinho's first choice long term. He's really intelligent and has great technique, but he can't really move well with the ball nor is he all that agile on it. Put him under some pressure and he loses it pretty often. Isco has huge potential, and brings the same type of creativity while being a lot more agile and skillful on the ball so he can move around in the ways Silva does for City, and perfectly capable of playing on the right of that midfield 3, with Pogba as a box to box and someone else (Carrick/Herrera) as the holding mid.
So far, I see our main starting lineup like this:
Ibrahimovic
Mart/Rash Pogba ??? Mkhitaryan
Carrick/Ander
Mata can come in that 3rd midfield role perfectly fine in easier games, but that would be too open against tougher teams probably. Isco can play a bit deeper then Mata, while being more aggressive and more mobile off of it, so that's why he would be perfect for all scenarios there. Can Griezmann come in and play that role? Not in my opinion, so Mkhitaryan would have to come inside and he would play as the right winger, which he can do but not what we would spend that money on him for. Like you say, he's a second striker while we need a 3rd midfield type playmaker like David Silva, and Isco would be available and not that expensive really.